The invention relates generally to a roll for the pressure treatment of webs of material, and more particularly to an improved damping device for reducing the vibratory motion of a hydraulically-supported roll or a conventional solid roll.
A roll of the type having a rotating hollow roll forming a working roll circumference at its outer diameter and a stationary cross piece extending through the hollow roll forming a clearance space therewith, with the cross piece having ends protruding from the hollow roll to which external forces may be applied for supporting the roll and means, including a liquid, for hydraulically supporting the hollow roll at the cross piece, constitutes a system which is able to vibrate with a number of degrees of freedom during operation of the roll as it rotates. The frequencies of the vibrations produced during rotation of the rotating hollow roll are approximately in the range of 80 Hz. when using a roll having a length of 4-10 m and roll diameter of 40-100 cm. The problem to which this invention is directed is the non-uniformity produced by the above-mentioned vibrations during treatment of the web of material.
This problem is especially prevalent in the wet press of a paper-making machine. In the wet press, these vibrations may lead to an irregular compaction pattern and, consequently, an irregular dewatering pattern. Thus, the paper webs have moisture contents which vary over the surface area of the web. Vibrations of the roller relative to the felt, i.e. vibrations in the plane transverse to the plane of the web, result in relatively large displacements and hence non-uniformities at the web center and web edges.
If the vibrations occur repetitively, i.e. one point of the web is exposed in every pass to a stronger compression in the roll gap than other points, the pattern formed in the web intensifies. Heretofore, one remedy to prevent this consisted of tensioning the web. This prevents areas of the web from getting into the roll gap rhythmically and exciting new vibrations further reinforcing their deviation from the adjacent regions.
In addition to the natural flexural vibrations of the cylinder and the cross head or cross piece of a hydraulically-supported roll causing non-uniform treatment of the web, displacements of the roll as a whole perpendicular to its axis also cause non-uniformities. An example of this is what is known as barring in a calender which is caused by the propagation of shocks through the roller stack when the rolls have equal diameters or diameters having a certain ratio to one another. This effect causes the appearance of cross stripes or bars in the treated paper. Previous solutions to this problem have been the selection of different roll diameters, lateral staggering of the rolls, and the provision of outer deflector rolls to avoid the simultaneous presence of vibration crests or vibration troughs in the roll gaps. This phenomenon is not limited to hydraulically-supported rolls having stationary cross heads and revolving cylinders but also occurs with conventional solid rotating rolls.
The problem of vibration of a roll causing non-uniform treatment of the web of material as it passes through the roll gap defined by a roll and a counter-roll has been recognized, as can be seen from DE-PS 15 61 706. This patent generally shows a roll for the pressure treatment of paper webs having a device for damping vibrations of the roll occurring during rotation thereof. The roll comprises a revolving cylinder and a stationary cross head piece extending lengthwise through the cylinder. The ends of the cross head protrude from the cylinder and form a roll neck for supporting the roll. Located within the cross head are piston-type pressure shoes which are positioned opposite each other in the active plane of the roll, i.e. in the working plane of the resultant of the forces exerted by the roller. The pistons can shift radially in cylinder-like grooves extending substantially over the working length of the roller to bear against the inner circumference of the cylinder. The grooves are filled with a pressurized liquid which forces the shoes against the inner circumference of the cylinder to create a largely uniform line pressure. Since the pressures in the mutually-opposite grooves act in opposite directions, the magnitude of the line pressure depends upon the difference between the pressures in the grooves. The cylinder spaces opposite each other in the active plane beneath the pressure shoes are connected by channels passing radially through the cross head which are arranged in close proximity to each other. The channels intersect a longitudinal bore in which a sealing piston is displaceable. Depending upon the position of the piston, one or more of the channels may be closed by the piston. The pressurized liquid can pass through the channels from one groove to another according to the pressure difference. Due to the small diameter of the channels, a throttle effect occurs based on the number of channels allowed to communicate by the piston. Thus, when vibrations occur, the throttle effect results in damping having an intensity which can be varied by opening or closing the channels.
One of the problems with the damping device in DE-PS 15 61 706 is that the damping effect actually attainable is small. The actual displacements of the cylinder relative to the cross head are very small, and correspondingly, only very small quantities of liquid pass through the channels. Thus, an appreciable throttle effect, and hence a noticeable damping, cannot be expected with this type of damping device.
DE-OS 33 06 838 also relates to the problems of vibration in rotating hollow rolls. This patent provides a damping element in the form of a liquid cushion positioned in the annular space between the cylinder and the cross head. The liquid cushion is formed by a piston bearing against the roll wall and moveable relative to the yoke. The piston separates the annular space from a liquid chamber containing the cushion liquid.
In a manner similar to DE-PS 15 61 706, one of the problems in DE-OS 33 06 838 is that the damping effect attainable is limited, because the damping is brought about by inappreciable fluid flow. Additionally, this type of device is not an effective solution to the damping problem, because it creates another problem or disadvantage in that the internal damping arrangement provided reduces the bending resistance of the cross head. The damping device provided is located in the region of the roll which produces the line pressure for supporting the hollow roll. Since accommodation must be made for the piston-type hydraulic elements in order to create the liquid cushion, valuable space or working area for supporting the cross head is lost, and hence the bending resistance of the cross head is diminished.